Automotive ride control systems provide active suspension response to road conditions by reacting to forces tending to compress the suspension members. In order to accomplish this, it is necessary to provide a real time measurement of the position of various portions of the suspension system, such as each wheel assembly, with respect to the vehicle frame. Those relative position variables are provided as an input to a ride control computer, which dynamically responds by adjusting the fluid pressure in the respective shock absorbers.
Both mechanical and electrical sensors have been used to measure the positions of the various portions of the suspension system. Among the advantages of electronic sensors are that they are easier to calibrate, less prone to wear and more compact, so that they are less exposed to deformation by gravel or ice dislodged from the road. Among the types of electronic sensors which find use in an automotive environment are variable resistance, variable reluctance, differential transformer and Hall Effect sensors.
One preferred location for an electronic position sensor is in or near the shock absorber, where the relative movement of each wheel may be monitored by the relative movement of the telescoping shock absorber. The environment inside a shock absorber is extremely hostile, subjecting an internally mounted sensor to pressures of up to 6500 psi and temperatures of up to 135.degree. C. Even when located outside the shock absorber, the sensor will be exposed to a severe environment requiring that the sensor be resistant to degradation by various automotive fluids and extremely stable under wide swings in ambient temperature. Additionally, to be successful in the automotive field, the sensor must be inexpensive to produce and reliable in performance, and must not be subject to significant wear over extended periods of time.
One drawback to the use of electronic sensors is that the electrical properties of the materials from which the sensors are constructed change with temperature. One prior art approach to temperature correction is to employ a temperature sensor coupled to a microprocessor capable of performing a temperature correction on the raw output of the sensor. While a sensor system incorporating an added temperature sensor has the advantage of increased accuracy, it has the disadvantage of increased complexity and expense to design and manufacture. Due to subtle differences between electronic sensors, it may be necessary to calibrate the temperature correction for each pair of position and temperature sensors separately, thereby significantly increasing the labor costs per unit.
U.S. Pat. No. 3,891,918 to Ellis concerns a measuring system having a transducer that produces a time varying function used to provide a direct digital measurement of a displacement.